No Mercy

B in general just gives you a lot more options. You have two (almost three, if you count connector from A->B) entrances for you to re-enter in the chance you lose a large number of players and chipped their numbers fairly well.

On the note of entrances - it's fairly easy for you to play up near catwalk if you have some form of disadvantage, then just work it slowly with respawns (if players have died). You don't really have an option with A - where are you going to sit while you wait for respawns? It's kind of all or nothing defending A, and that kind of dice-roll play is just plain shit for consistency (and in general, really).

A few other things:

-Roof at B is a really good place to hold as it is high ground (no shit) making it easier for you to spam them, in turn hard for them to damage you if you have players up there. Rotate soldiers on there like it's badlands spire!

-It's pretty easy to somewhat take ground for free at B, but on that same token it is really easy for you to just walk into them with a single uber, and they're pretty much committed and will definitely drop a few players whilst you will be able to have extremely minimal casualties (assuming you play it properly!).

-Sentry positions are a lot more "adaptable" I guess you could say - whereas at A the positions for sentry guns are pretty shit (from what I remember, I may be wrong about this). At B you have rock (really old, but can still work), front medkit (great for denying jumpers/aggressive holding) or the "enigma sentry" (behind point, also good for controlling roof/close C->B entrance). A lot of different sentry placements for different playstyles is always good!

-This kinda goes back to one of my previous points about A having more entrances - the point at b is A LOT closer to RED spawn. Getting to A, you would have to add an extra 5+ seconds if you were to simply walk there. You already have slower spawns since you are the defending team - why add unnecessary time?

-A demoman on B point is terrifying. It is really hard to damage him/get him out of there and it is so easy for him to shut down any spawn side attacks if you don't approach it carefully. Be wary!

Only reasons you might want to defend A is because:
- You want to run 2 scouts that will dominate your opponents if given room to move; there is more room at A.
- You are really betting your whole team defense on a strong forward hold at C with hut sentry. Holding A and consequently retreating to C via the A-C connector gives an slightly easier transition into such a hold.

Alternatively you could be trying to iron-man hold (no idea why its called that) where you rotate between A and B and attempt to defend both for as long as possible.

No Mercy

Also isn't it really easy to get cut off from A and spawn since it takes about 10 seconds to walk from blue spawn to ac connector, whereas it'd take longer to walk from red spawn there? Spawn advantage would just decimate you.